IE8对象当对象包含null值时应出现错误

我正在使用一个小的多边形填充，以便使Object.key可用于IE8。当我的对象包含所有值时，一切似乎都很正常。然而，当我在对象中有空值时，我会得到一个关于预期对象的错误。我不知道为什么有一个null会很重要。我的代码如下：

  var data = [{
     x: Date.UTC(2014, 8, 1),
     y: 730,
     pf: ["001", "002"],
     nf: ["001", "002"],
     t: ["001", "002"],
     im:["001"]
  }, {
     x: Date.UTC(2014, 9, 1),
     y: 742,
     pf: ["001", "002"],
     nf: ["001", "002"],
     t: ["001", "002"],
     im:["001"]
  }, {
     x: Date.UTC(2014, 10, 1),
     y: 746,
     pf: ["001", "002"],
     nf: ["001", "002"],
     t: ["001", "002"],
     im:["001"]
  }, {
     x: Date.UTC(2014, 11, 1),
     y: null
  }, {
     x: Date.UTC(2015, 0, 1),
     y: 737,
     pf: ["001", "002"],
     nf: ["001", "002"],
     t: ["001", "002"],
     im:["001"]
  }, {
     x: Date.UTC(2015, 1, 1),
     y: 751,
     pf: ["001", "002"],
     nf: ["001", "002"],
     t: ["001", "002"],
     im:["001"]
  }, {
     x: Date.UTC(2015, 2, 1),
     y: 765,
     pf: ["001", "002"],
     nf: ["001", "002"],
     t: ["001", "002"],
     im:["001"]
  }, {
     x: Date.UTC(2015, 3, 1),
     y: 766,
     pf: ["001", "002"],
     nf: ["001", "002"],
     t: ["001", "002"],
     im:["001"]
  }, {
     x: Date.UTC(2015, 4, 1),
     y: 757,
     pf: ["001", "002"],
     nf: ["001", "002"],
     t: ["001", "002"],
     im:["001"]
  }, {
     x: Date.UTC(2015, 5, 1),
     y: 750,
     pf: ["001", "002"],
     nf: ["001", "002"],
     t: ["001", "002"],
     im:["001"]
  }, {
     x: Date.UTC(2015, 6, 1),
     y: 748,
     pf: ["001", "002"],
     nf: ["001", "002"],
     t: ["001", "002"],
     im:["001"]
  }, {
     x: Date.UTC(2015, 7, 1),
     y: 723,
     pf: ["001", "003"],
     nf: ["001", "002"],
     t: ["001", "002"],
     im:["001"]
  }]

  var pfMap = {
     "001": {
        "sd": "HI",
        "ld": "By"
     },
     "002": {
        "sd": "HI",
        "ld": "By"
     }
  }
  var nfMap = {
     "001": {
        "sd": "HI",
        "ld": "By"
     }
  }
  var tMap = {
     "001": "Hi",
     "002": "by"
  }
  var imMap={
      "001":"HI"
  }
  // Shim added to aid in IE8
  if (!Object.keys) {
    Object.keys = (function() {
      'use strict';
      var hasOwnProperty = Object.prototype.hasOwnProperty,
          hasDontEnumBug = !({ toString: null }).propertyIsEnumerable('toString'),
          dontEnums = [
            'toString',
            'toLocaleString',
            'valueOf',
            'hasOwnProperty',
            'isPrototypeOf',
            'propertyIsEnumerable',
            'constructor'
          ],
          dontEnumsLength = dontEnums.length;
      return function(obj) {
        if (typeof obj !== 'object' && (typeof obj !== 'function' || obj === null)) {
          throw new TypeError('Object.keys called on non-object');
        }
        var result = [], prop, i;
        for (prop in obj) {
          if (hasOwnProperty.call(obj, prop)) {
            result.push(prop);
          }
        }
        if (hasDontEnumBug) {
          for (i = 0; i < dontEnumsLength; i++) {
            if (hasOwnProperty.call(obj, dontEnums[i])) {
              result.push(dontEnums[i]);
            }
          }
        }
        return result;
      };
    }());
  }
  //recursion
  function traverseData(data, elementName) {
     var keys = Object.keys(data);
     for (var i = 0; i < keys.length; ++i) {
        if (Object.prototype.toString.call(data[keys[i]]) == "[object Object]" || Object.prototype.toString.call(data[keys[i]]) == "[object Array]") {
           traverseData(data[keys[i]], keys[i]);
        } else {
           if (elementName != null && window[elementName + "Map"]) {
              data[keys[i]] = window[elementName + "Map"][data[keys[i]]];
           }
        }
     }
   }
 traverseData(data, "data");
 var end = data;

正如你所看到的，这是一个简单的递归，我正在构建它来创建一个新的图表对象。如果您注意到数据对象上的第4个条目的y值设置为null。如果我在这里填写一个数字，一切都会按预期进行。然而，如果它被保留为null，我会得到错误。知道为什么吗？

编辑：在polyfill中抛出错误的有问题的行。我有它在块吹评论

        if (hasDontEnumBug) {
          for (i = 0; i < dontEnumsLength; i++) {
            if (hasOwnProperty.call(obj, dontEnums[i])) { //  Throws error here
              result.push(dontEnums[i]);
            }
          }
        }